Photolithography is one of the process steps commonly encountered in the processing of silicon wafers for semiconductor applications. In photolithography, a wafer surface with a deposit of silicon nitride is coated over with a light-sensitive liquid polymer or photoresist and then selectively exposed to a source of radiation using a template with a desired pattern. Typically, ultraviolet light is shone through or reflected off a surface of a mask or reticle to project the desired pattern onto the photoresist covered wafer. The portion of the photoresist exposed to the light is chemically modified and remains unaffected when the wafer is subsequently subjected to a chemical media that removes the unexposed photoresist leaving the modified photoresist on the wafer in the exact shape of the pattern on the mask. Typically, the wafer is subjected to an etch process that removes the exposed portion of the nitride layer leaving a nitride pattern on the wafer in the exact design of the mask.
The industry trend is towards the production of chips that are smaller and/or with a higher logic density necessitating smaller and smaller line widths on larger wafers. Clearly, the degree of fineness to which the surface of the reticle can be patterned, and the degree to which this pattern can be faithfully replicated onto the wafer surface, are factors that impact the quality of the ultimate semiconductor product. The resolution with which the pattern can be reproduced on the wafer surface depends on the wavelength of ultraviolet light used to project the pattern onto the surface of the photoresist-coated wafer. State-of-the-art photolithography tools use deep ultraviolet light with wavelengths of 193 nm, which allow minimum feature sizes on the order of 100 nm. Tools currently being developed use 157 nm Extreme Ultraviolet (EUV) light to permit resolution of features at sizes below 70 nm.
The reticle is a very flat glass plate that contains the patterns to be reproduced on the wafer. Typical reticle substrate material is quartz. Because of the tiny size of the critical elements of modern integrated circuits, it is essential that the operative surface of the reticle (i.e. the patterned surface) be kept free of contaminants that could either damage the surface or distort the image projected onto the photoresist layer during processing thereby leading to a final product of unacceptable quality. Typically, the critical particle sizes are 0.1 μm and 0.03 μm for the non-patterned and patterned surfaces respectively when EUV is part of the photolithography process. Typically, the patterned surface of the reticle is coated with a thin, optically transparent film, typically of nitrocellulose, attached to and supported by a frame, and attached to the reticle. Its purpose is to seal out contaminants and reduce printed defects potentially caused by such contaminants migrating to the image plane. However, extreme EUV utilizes reflection from the patterned surface as opposed to transmission through the reticle characteristic of deep ultraviolet light photolithography. Currently, the art does not provide pellicle materials that are transparent to EUV. Consequently, the reflective photomask (reticle) employed in EUV photolithography is susceptible to contamination and damage to a far greater degree than reticles used in conventional photolithography. This situation imposes heightened functional requirements on any container designed to receive, store, transport and ship a reticle destined for EUV photolithography use.
Clearly, unnecessary and unintended contact with other surfaces during manufacturing, processing, shipping, handling, transport or storage is highly undesirable in view of the susceptibility of the delicate features on the patterned surface of the reticle to damage due to sliding friction and abrasion. Secondly, any particulate contamination of the surface of the reticle will likely compromise the reticle to a degree sufficient to seriously affect any end product obtained from the use of such a reticle during processing. Particles can be generated within the controlled environment containing the reticle during processing, transport and shipping. Sliding friction and consequent abrasion is one source of contaminating particulates. A reticle sliding from its desired position in a reticle container during transport, for example, is another source of particulates. Such an out-of-position reticle will also likely be misaligned when automatically retrieved from the container and positioned into processing equipment potentially leading to an end product that is of unpredictable quality. Sliding contact during placement and removal of a reticle from the container to the lithography equipment also creates opportunities for particulate generation and contamination. Finally, shock and vibration of the container can be transmitted to the reticle and components holding the reticle causing friction and associated particle generation.
Conventionally, reticles are shipped to the fabrication facility in which they are used in one container and are stored in the fabrication facility in between uses in other containers. The shipping containing is typically discarded after use. The transfer of the reticles from the shipping containers to the containers in which they are stored within the fabrication facility creates another opportunity for contamination. Conventional requirements for shippers for reticles and containers for use within the fabrication facility are dramatically different. Combining the container for both uses would eliminate the opportunity of incursion and generation of particulates during the transfer from the shipper container to the fabrication facility use container but presents significant design challenges. For example the container would need to be able to handle the potential dramatic changes in atmospheric pressure during transportation, such as associated with altitude and temperature changes. Also shock absorption capabilities in transportation are much more demanding than in the controlled robotic transfers occurring in fabrication facilities.
Some of the considerations discussed above are also applicable to semiconductor wafer substrates. Recognizing the need for a controlled environment around the wafer, especially during storage, processing and transport, prior art has evolved approaches to isolation technology that allows for control of the environment in the immediate vicinity of a wafer by providing for a container to house the wafer so that it can be kept relatively free from incursion of particulate matter.
Wafers are typically shipped to a fabrication facility in a shipping container and then transferred to a separate container for storing the wafers in between processing steps in the fabrication facility. 200 mm wafers are typically shipped in sealed plastic “shippers” either edge supported in a spaced array or stacked vertically with sheet material spacers in “coin stack wafer shippers” Industry standardized containers for holding 200 mm wafers in between processing steps in fabrication facilities are known as standard mechanical interface pods, or SMIF pods and having bottom opening doors. For 300 mm wafers, the shippers are known as front opening shipping boxes, or FOSBS, and the containers for holding the wafer in between process steps are known as front opening unified pods, or FOUPS. Reticles stored in Fabrication facilities in between fabrication steps now often are stored in bottom opening containers similar to the standardized SMIF pods and are termed reticle SMIP pods, or RSPs.
Even when substrates, that is wafers and reticles, are in such a controlled environments, particulates that may be present inside the controlled environment can migrate due to pressure changes of the air trapped in the controlled environment or turbulence of the trapped air brought on by rapid movements of the container and/or by disturbing the trapped air volume, such as by simply opening and closing the container. Also, thin walled shippers or FOSBS may experience wall movement due to altitude related pressure changes causing the trapped air inside the controlled environment to be displaced. Temperature changes can set up convection currents within the container. Dimensional changes of the container and its components can compromise the functioning of the support and retaining mechanisms leading to wafer misalignment and/or warping of the substrate carried within the container. Dimensional changes of the container wall due to pressure fluctuations can compromise the sealing between the cover and the door of the carrier allowing particulate incursion within the carrier.
Prior art approaches, particularly in wafer containers, utilize a breathing apparatus between the external environment and the internal controlled volume of air. The breathing apparatus provides a path for the air to flow. A filter interposed in the path is expected to provide a barrier to incursion of particulates from the external environment into the controlled environment of the carrier. However, as noted above, the reticle used in a EUV photolithography process has very fine and delicate features so the critical particle sizes are only of the order of 0.1 μm and 0.03 μm for the non-patterned and patterned surfaces of the reticle respectively. At such low particle sizes, a filter would require a very fine pore size causing a considerable resistance to fluid flow across it thereby necessitating a larger filter surface area. The alternative to a larger filter surface area is a slower response to sudden pressure changes such as those encountered in shipping the container. Both of these are not desirable alternatives because one of the objectives of reticle SMIF pod design is to keep the controlled volume to a minimal so it can be effectively sealed against incursion of particulates. Minimizing the controlled volume within which the reticle is positioned whilst providing for a large filter area to achieve pressure equalization within the controlled volume are incompatible objectives.
It is desirable that particulates that are generated or are otherwise introduced or present within the controlled environment are prevented from settling on the reticle. In this regard, it is preferable to have a minimal volume for the environment within which the reticle is carried and which has to be controlled to avoid particulate contamination. It is also desirable that the air in the controlled volume remains relatively static. For example, the deflection of a wall of the container in response to large and sudden pressure differences can induce a pressure wave inside the controlled volume forcing particulates into suspension and subsequent migration.
What is needed is a substrate container suitable for use both as a shipper and to store the substrates in between processing steps in the fabrication facilities. What is needed is a container that provides improved resistance to particulate generation and minimization of particle disruption or movement within a substrate container during transportation, and opening and closing of the container. What is needed is a pressure equalization system that effectively equalizes pressure between an internal controlled environment of the carrier and the air external to the carrier without incursion or excursion of air from the controlled environment and with minimal turbulence of the air already present within the controlled environment.